1. Field ofthe Invention
The invention concerns an axial flow turbine with reaction blading whose outlet rotor blades with high Mach number flow are followed by a diffuser with axial outlet into an exhaust gas pipe. Such systems are especially used in gas turbine construction. Generally speaking, the axial exhaust pipe emerges into a chimney through which the turbine exhaust gases are released into the atmosphere.
2. Description of the Prior Art
Because of the increase in volume of the exhaust gases, due to their expansion when flowing through the usually multi-stage turbine, the blading lengths of the guide vanes and rotor blades are matched to the changes in density. This produces a conical flow duct in which, depending on the type of design, both the inner boundary wall, i.e. the hub, and the outer boundary wall, i.e. the cylinder, may be inclined at a certain angle to the centre-line of the machine. In many designs, the hub is cylindrical with corresponding angular adaptation of the cylinder. In machines in which high Mach number flow occurs, the angle between the hub and the cylinder can easily attain 30.degree. or more. As a consequence, the meridianal streamlines at the blading outlet extend over this angular rage. The diffuser for recovering the kinetic energy is downstream of this outlet. If the conicity were to be continued in a straight line, the angle mentioned (30.degree.) would be completely unsuitable for retarding the flow and achieving the desired increase in pressure. The flow would separate from the walls.
The turbine designer knows that a diffuser angle of about 7.degree. should not be exceeded. As a result, he will reduce the angle of 30.degree. mentioned to 7.degree. and connect the diffuser determined in this manner on the basis of practical considerations.
Investigations have shown that a diffuser with an axial outlet designed in this manner is unsuitable. The deflection of the streamlines at the kink positions of the diffuser inlet and the associated undesirable buildup of pressure reduces the drop, i.e. the gas works over the blading. This results in decreased power. The energy not employed leads to local excess velocities at the diffuser outlet and these are subsequently dissipated in the outlet gas pipe.